Interesting stuff. It seems a bit like 'horses for courses' guys. I suppose the main difference between car and aero engines is their operating speed and the fact that car engines run with a flywheel rather than a prop. A general engineering principal for those using machine tools - lathe surface grinders, millers - is that a high speed and light cut gives the best finish. Assuming the analogy follows, and as far as I can see the only differences are those of scale - i.e. the depth of material you are intending to remove, a car engine off-load would satisfy that criteria. Using that analogy a car engine (or helicopter?) should achieve the best running fits. This, however doesn't take into account heat cycling which seems to be one thing we are all in agreement on.

I guess you could summarise and say that if you keep the revs up and the load down, stop the engine and allow to cool when the temp increases and gradually extend the process to full speed, on load operation you will not be far away from ideal fits.

I suppose it's one of those things that's down to experience and the intended use of the engine. Watch an experienced operator "run in" a speed engine - it's frightening the revs they use.

What I find most interesting is that everyone you speak to has a different opinion on how to run in engines: what sorts out the men from the boys is that little bit more horsepower and those last few revs.

Mick